This invention relates to electronic compositions, and more particularly, to metallizations useful in producing conductors on dielectric substrates.
Metallizations which are fired onto ceramic dielectric substrates to produce conductor patterns usually comprise finely divided noble metals and an inorganic binder, and usually are applied to the substrate as a dispersion of the inorganic powders in an inert liquid vehicle. Upon firing (usually in the range 700.degree.-1000.degree. C.), sintering occurs and the metallic component provides the functional (conductive) utility, while the binder (e.g., glass, Bi.sub.2 O.sub.3, etc.) bonds metal particles to the substrate and to one another.
There is a need for conductor compositions in which noble metals are replaced by base metals such as copper, yet having good performance characteristics. Copper compositions are usually fired in an inert, nonoxidizing atmosphere (such as nitrogen) to prevent copper from reacting with oxygen in the air at elevated temperature. Some previous attempts to make copper compositions have used typical glass binders such as those having high bismuth oxide, cadmium oxide, or lead oxide contents. However, it has not been generally recognized that in the absence of air during firing at 700.degree.-1000.degree. C. copper metal can be oxidized to cuprous oxide, or further to cupric oxide, by reduction of some of the constituents of the glass binder. Bismuth oxide and cadmium oxide have been found to be particularly susceptible to reduction under such conditions, lead oxide being less susceptible. Accompanying the oxidation of copper metal to an oxide of copper is the reduction of bismuth oxide, cadmium oxide or lead oxide to the corresponding metal.
When copper metal is oxidized, the oxide is not solderable and leads to spots in the resultant fired metallization which are not soldered by typical solders such as Sn/Pb. Although not intended to be limiting, it seems that the reaction of copper with bismuth oxide is generally preceded by separation of the glass binder into two phases. One is a bismuth oxide rich phase which is absorbed by the copper metal surface. The remaining phase, often a lead borosilicate glass, does not wet the absorbed bismuth oxide phase and adhesion of the metallization is reduced. Tensile strength tests often show the specimens failing in the metallization itself. Where the absorbed layer of bismuth oxide phase has been reduced to bismuth metal during firing, the metallizations tend to fail by loss of adhesion to the substrate.
The above behavior is most inconvenient since Bi.sub.2 O.sub.3, CdO and PbO are common constituents in producing glasses for metallizing compositions, in that they produce glasses of low viscosity and softening point, which wet the typical alumina dielectric substrates at the relatively low temperatures typically used in firing film metallization on substrates. They further provide useful levels of solderability and adhesion to the substrate.